This invention relates to the technical field of diagnosing electrical systems (particularly high-voltage systems) by detecting and then processing partial electrical discharges.
It should be noted that a partial discharge is an electric discharge limited to a portion of the insulation of an electrical system and does not therefore cause immediate failure of the system but its gradual degradation. By their very nature, therefore, partial discharges are substantially limited in extent to a defect in the insulating system.
In light of this, diagnostic methods based on the detection and interpretation of partial discharges are among the most promising and widely studied in the context of scientific research since the study of partial discharges makes it possible to investigate the nature of defects in the insulating system where the discharges occur.
The detection and subsequent processing of partial discharges for diagnostic purposes, however, has not yet become a standard industrial tool for planning the maintenance and/or substitution of high-voltage electrical components on account of the difficulties encountered in interpreting the results of measurements.
As regards the detection of partial discharges, several methods have been developed, based on the use of different physical phenomena associated with discharges, such as, for example, methods of optical, acoustic and electrical type.
This invention relates in particular but not exclusively to detection methods of the electrical type which, as is known, involve measuring the current pulses that travel a detection circuit coupled to the electrical system being checked.
These detected current pulses (hereinafter referred to, for convenience, as discharge pulses) have a time profile that depends on the dynamics of the partial discharges (that is, on the physics of the discharge phenomena) and on the nature of the means which the detected pulses cross as they travel from the discharge site (where the discharges occur) to the detection site.
The time profile of the discharge pulses, consisting of the waveform of the pulses themselves, contains precious diagnostic information regarding both the physical phenomena associated with the discharges (correlated with the nature of the defects of the insulating system) and the nature of the medium which the detected pulses travel through (correlated with the location of the defects in the insulating system).
When measuring partial discharges, it is important to assign to each signal detected a value of a phase parameter representative of the phase of the voltage applied to the electric apparatus at the time of detecting the signal (for example using a sync probe positioned along the electric apparatus).
It should be noted that by assigning such a value it is assumed that the electric apparatus is subjected to alternating current (AC) voltage. In effect, according to a known technique, the phase angle of the voltage at the time a signal is detected is measured and that angle is assigned to the signal itself.
The phase parameter is commonly used in combination with an amplitude parameter (correlated with the intensity of the discharge pulses detected) in order to interpret the partial discharge measurement data for diagnostic purposes.
In effect, it is known that the discharge pulses lying in a plane having as its coordinates the amplitude and phase parameters form a pattern (also known as PRPD pattern, or Phase Resolved Partial Discharge pattern) which well represents the nature of the discharge site, that is, of the defect where the partial discharges occur.
As to the difficulties of interpreting the results of partial discharge measurements, these depend not only on the need to have experience and a specific case history but also on the fact that the measured data may be unreliable or insignificant.
In light of this, the problems that invalidate diagnostics based on the measurement of partial discharges are essentially of two kinds:                during detection of the signals associated with the partial discharges, information essential for subsequent processing of the signals themselves for diagnostic purposes may be lost (loss of information might consist, for example, of failure to detect a pulse, or assigning to a certain partial discharge activity a signal from another source or relating to a different discharge activity);        during such detection, noise may be superposed on the discharge signals or the signals from different sources may be superposed on each other, thus making it objectively difficult to interpret the results since it is impossible to perform significant statistical processes on data which is not uniform and/or not pertinent to the individual partial discharges to be processed.        
As regards the loss of information during detection, it should be noted that signals associated with partial discharges are electric pulses having a very high frequency content (with leading edges in the order of nanoseconds or dozens of nanoseconds) and, in some cases, have a relatively high repetition frequency (for example hundreds or thousands of pulses per second).
Thus, with regard to the instrument used to detect the signals associated with the partial discharges, the problem is that of capturing very rapidly and efficiently electrical signals having a high frequency content, while maintaining the information content of the signals as much as possible. Furthermore, the instrument should allow the significant diagnostic signals to be effectively separated from noise or other “unwanted” signals.
The solution to the above mentioned problems is particularly difficult if we consider the need to detect the partial discharges and to process the state of the electric system in unattended manner or with the minimum of operator intervention (in some cases, such as online monitoring systems, there is no operator at all).
In practice, if unwanted signals (for example, signals associated with noise or discharge activities outside the measuring instrument) are superposed on the partial discharge activities to be detected, there is the risk that many of the discharge signals to be detected will remain undetected by the instrument, especially if the repetition rate of the unwanted signals is very high.
In the context of partial discharge detection instruments currently in use, known in the prior art (from document WO2007/144789 in the name of the same Applicant as this invention) is the employment of dedicated hardware configured to detect the discharge signals and to extract the pertinent parameters from them in real time.
In particular, that instrument comprises:                an input stage set up to receive a discharge signal representative of one or more partial discharge pulses and a sync signal representative of an alternating voltage applied to the electric apparatus;        an output stage set up to transfer data in digital form to the output from the instrument;        a data processing stage connected both to the input stage and to the output stage for receiving the discharge signal and the sync signal, and extracting substantially in real time for each pulse detected the value of an amplitude parameter correlated with a pulse amplitude, and the value of a phase parameter representative of the phase of the voltage applied to the electric apparatus concurrently with the pulse, and transferring to the output stage a processed digital signal comprising the values extracted.        
This instrument therefore increases detection speed considerably.
It does not, however, fully solve the above mentioned problems.
In effect, unwanted signals with particularly high repetition rates are sometimes superposed on the discharge activities to be monitored. Indeed, in some cases, the unwanted signals have a waveform that is similar to (or at least easily-mistaken for) that of the signals to be detected.
Document WO2007/144789 also describes a conditioning element designed to digitally filter the processed signal as a function of the derived values.
The conditioning element filters the measured signal according to predetermined parameters (correlated with diagnostic processes on the electrical system based on the processing of the partial discharge activity).
More specifically, WO2007/144789 describes how the conditioning element advantageously makes it possible to reject noise or select pulses belonging to a given discharge phenomenon with respect to other superposed pulses, in real time, on the basis of the waveform of the signals.
Document WO2007/144789, however, provides no further indication on how the conditioning element is made and works.
More specifically, WO2007/144789 does not provide any teachings or suggestions on possible filtering strategies, but very simply and generically refers to a possibility of selecting some pulses and rejecting others based on the waveform of the signals detected.
Furthermore, during the monitoring of a partial discharge activity, it is often necessary to focus on the signals relating to a predetermined discharge activity (correlated with a corresponding defect in the insulating system being checked) ignoring all other signals.
This problem is neither tackled nor solved by WO2007/144789.
Document WO2009/013639, in the name of the same Applicant as this invention, regards a method for detecting, identifying and locating partial discharges occurring at a discharge site along an electric apparatus such as, for example, a cable.
According to that document, the signals are separated as a function of detected parameters. Separation does not, however, take place in real time, when the signals themselves are detected, but during a subsequent step of processing data which has been stored.
Hence, the method of document WO2009/013639 does not allow real time filtering but aims to identify a strategy for locating the discharge sources based on the processing of data groups acquired on different sections of the electric apparatus being checked.
Further, document EP1094323 discloses a method and a system for identifying the source of a partial discharge activity by processing previously stored data, in particular using mathematical algorithms based on fuzzy logic.
That document does not regard either real-time detection or real-time filtering of the signals.
Thus, even document EP1094323 does not tackle the above mentioned problems.